Where are you from? I can't say I've ever booked in advance from overseas. But the touch screen kiosks that most of the train stations are quick and easy.
If you're an American, a britrail pass may be cheaper. Even for those two trips, it could be cheaper.
If I were you though I would consider the National Express(bus/coach). Those distances are quite short, and the National Express is quite reliable.
US Soccer and Toronto FC Fanatic.
July 23 - Sept 13: Back in Europe! Off to Brest, France for a French language course. Schedule is open before August 7 and after August 28, suggestions?